Generally, it can be said that the quality of a sound signal (word, phrase, a talking voice) synthesized by connecting compilation of sound elements, i.e. words, syllables, or shorter sound segments is determined by processing of the junction of the sound elements that are the constitution units of a sound. For example, an abrupt change of the waveform occurring at the junction, i.e. the discontinuity of the waveform becomes a cause of a harmonic noise, which degrades a signal to noise ratio of a synthesized sound and the intelligibility. It is also known that a fluctuation of the pitch frequency which is the fundamental frequency of the vocal chords deteriorates the naturalness of a synthesized sound. The auditory sensation of a human being is extremely sensible with respect to the fluctuation of the pitch frequency (the limit of perception is allegedly 0.1 percent) and the discontinuity of the pitch frequency of the connected sound elements makes a synthesized sound offensive and unnatural.
FIG. 1 is a block diagram showing a conventional time axis expanding apparatus. Referring to FIG. 1, the reference numeral 1 denotes a sound input terminal, the reference numeral 2 denotes an output terminal, the reference numerals 3 and 4 denote N-bit analog shift registers of such as BBD, and the reference numeral 5 denotes a low-pass filter (LPF). The reference numerals 6, 7, 8 and 9 denote analog switches, which serves to controllably switch a sound signal being fed from the input terminal 1 through the analog shift register 3 or 4 and the low-pass filter 5 to the output terminal 2. These analog switches are adapted to be on/off controlled, as shown, responsive to the Q and Q outputs of a frequency divider 11 which frequency divides at 2mN (m will be described subsequently) the output of a write clock generator 10 for the analog shift registers 3 and 4.
The analog shift registers 3 and 4 are write clock controlled alternately responsive through OR gates 14 and 15 to the AND gates 12 and 13 of the clock generator 10 and the Q and Q outputs of the frequency divider 11, and read clock controlled alternately responsive through the same OR gates 14 and 15 to the AND gates 17 and 18 of the read clock generator 16 and the Q and Q outputs of the frequency divider 11. More specifically, a sound signal applied to the input terminal the time axis of which has been compressed by m times (m&gt;1), for example, (such compressed signal is obtained by increasing the reproduction speed of a tape recorder by m times as compared with the recording speed, for example) is written into the analog shift register 4 through the analog switch 8 when the Q output of the frequency divider 11 is the logic one. The bit number of the shift register is N and accordingly if the input sound signal is sequentially loaded as a sampled train of the number mN, the trailing end portion of the number N of the sampled train of the number mN is stored in the shift register, the Q output of the frequency divider 11 is reversed to the logic zero, whereby the switch 8 is interrupted. At the same time the Q output of the frequency divider becomes the logic one, whereby the switch 6 is conducted, whereupon the analog shift register 3 effects a write operation in the same manner. As seen from the structure shown in the figure, the analog shift register 4 is clocked at that time by the read clock generator 16 and a read operation is achieved through the switch 9 controlled responsive to the Q output in the same manner. During the write period of the analog shift register 3 the other analog shift register 4 thus effects a read operation, whereupon when the Q and Q outputs of the frequency divider 11 are reversed again the analog shift register 4 effects a write operation and the analog shift register 3 effects a read operation. Now assuming that the clock frequency of the write clock generator 10 is f.sub.1, and the clock frequency of the read clock generator 16 is f.sub.2 and the respective clock frequencies are determined to satisfy the following equation: EQU f.sub.1 /f.sub.2 =m (1)
then the time axis is expanded by m times and the compressed sound as inputted to the sound input terminal 1 appears at the output terminal 2 with the time axis regained. Naturally, the read clock frequency f.sub.2 is determined to satisfy a Nyquist sampling theory with respect to a necessary output sound frequency band.
With the above described conventional apparatus, the jointing timing of the sound elements alternately outputted from the analog shift registers 3 and 4 is automatically determined per mN/f.sub.1 second responsive to the output of the frequency divider 11 for frequency dividing the write clock 10 by the factor 2mN. Therefore, a discontinuous waveform variation and a fluctuation of the pitch frequency are caused at the junction of the sound elements, as shown in FIG. 2. As described previously, the discontinuity of the waveform and the pitch at the junction of the sound elements considerably degrades the sound quality and the intelligibility.